If you only want to draw them by hand, as you were on a blackboard you can use any simple paint program, there are several free ones.
Take a look ...
Word and Powerpoint or their free rivals such as LibreOffice or OpenOffice have editors for quite rich formulas. They are very easy, but typographic control in LaTeX style is zero.
Structure diagrams need a drawing program. Office products have something quite clunky but easy to learn. Inkscape is the complex but capable one freebie.
Check this molecule ...
I know there's an open-source extension to LATEX specifically for chemical formulae - I think it's called chemfig - here's what I just found on quick lookup:
Overleaf - Chemfig
Also, for simple diagrammatic stuff, if you don't feel you need a chemistry formula specific tool, and general graphics / vector drawing works for you, then bear in mind that ...
You could use the Adobe Illustrator Draw app. It works on Android devices, and is available in the Google Play store.
To add an image, begin a new project. Tap the Layers icon, tap the + icon to add a new layer. Choose "Image Layer", select an image from your gallery or take a photo, or use select one from your cloud files, etc.
If you tap on the image ...
You could check the image orientation in Windows by right clicking an image file, select Properties, and open the Details tab. Not sure if you're using a Mac but I would suspect there will be something similar.
The Dimensions property will show the horizontal side (on the left) and the vertical side (on the right). So, if the horizontal size is the biggest ...
Probably one option is using blender directly.
Motion capture info
Position and skeleton
Render, green screen or even composite a 3D environment or doing camera tracking
Realtime rendering, Not as powerful as a gaming engine, but pretty interesting.
Open source, so you could tweek some things if you need or can.
For the models you could ...
For the resizing part: you can use a script from this answer as a part of an Action that'd crop/resize your image to square: https://graphicdesign.stackexchange.com/a/134135/125506;
For background part simply resize and gaussian/average blur your layer as a different step in the action;
And then use File > Automate > Batch to run this action on a ...
Stardew Valley developer concernedApe has said that he drew that game's pixel art in Paint.net. It works on Win10 and is free to use too (you can pay extra for getting it on the windows store, but you don't have to).
It doesn't have any animation-specific features, but seeing what Stardew has achieved, it's defintely possible to do great graphics in it.